Semiconductor laser

ABSTRACT

A buried heterojunction semiconductor laser appropriate for integration with other electronic circuitry and method of producing same, in which the width of a central stripe of the active region can be reduced beyond the physical size limitations of the connecting electrode so as to allow the semiconductor laser to oscillate in a stable manner and with low threshold current. The semiconductor laser is provided with a portion of the surface of the upper cladding layer located above the disordered active layer regions electrically connected with the upper cladding layer located above the nondisordered central stripe. As a result, the central stripe electrode can be of a width larger than that of the central stripe itself.

FIELD OF THE INVENTION

The present invention relates to a semiconductor laser and a method ofproducing same, and more particularly to a semiconductor laser suitablefor integration with electronic circuitry.

BACKGROUND OF THE INVENTION

In buried heterojunction semiconductor lasers of the multiple quantumwell type, each quantum well consists of a comparatively low band gaplayer, also called the well, sandwiched between a pair of barriers ofhigher band gap. When the plurality of quantum wells is very large, itis called a superlattice. Typically, such buried heterojunction laserswere fabricated to be discrete devices, and this allowed the p and nelectrodes to be located on opposite sides of the substrate. As aresult, the size of the electrodes could be relatively large and was notlimited by the width of the active region. In order to facilitateintegration of these devices with other electronic circuitry, some priorart heterojunction lasers are designed with both electrodes on the sameside of the substrate. However, this configuration implies that theelectrode which overlies the laser active region, and hereinafter calledthe central stripe electrode, be narrower than the active region andaccurately positioned thereon.

FIG. 2 shows a cross-section of a prior art heterojunction laserappropriate for integration. In FIG. 2, a p-type AlGaAs cladding layer202, multiquantum-well (MQW) active layer 203, n-type AlGaAs claddinglayer 204 and n-type GaAs contact layer 205 are successively produced ona semi-insulative GaAs substrate 201. Zn is then selectively diffused tocreate p-type diffused regions 208 such that an n-type region, in astripe configuration, remains. In addition, in the area where the p-njunction is exposed to the surface, the n-type GaAs contact layer isselectively etched so that p side and n side electrodes 206 and 207 canbe produced on the surface of the p-type and n-type regions,respectively.

According to this production method, the MQW active layer 203 isdisordered at the Zn diffused areas 208 and becomes an AlGaAs layer ofaverage composition, thereby creating a buried heterojunction laserstructure. The operation of this prior art semiconductor laser isdescribed in the following paragraph.

In such a semiconductor laser, two kinds of p-n junctions are provided.The first kind is created at the periphery of active region 209 (wherethe MQW is not disordered). The second kind of junction is createdbetween the n-type AlGaAs cladding layer 204 located above active region209 and each diffused region 208. Because the first kind of p-n junctionhas a diffusion voltage lower than that of the second kind, when avoltage is applied between the p side and n side electrodes, a currentflows through the p-n junction located at the periphery of active region209. As a result, carriers are injected in the active region. Sinceactive region 209 is adjacent on its four sides to AlGaAs having a lowrefractive index, it becomes a light waveguide, and if the width ofactive region 209 can be made narrow enough, the laser will oscillate ata stable single mode with a low threshold current. Finally, because thisprior art semiconductor laser has both p and n side electrodes locatedon the same main surface with little step difference, it is therefore ina form appropriate for integration.

In prior art buried heterojunction lasers which are appropriate forintegration and which utilize the disordering of a superlattice, the nside electrode is confined to the width of the active region. Referringto FIG. 2, it will be appreciated that if the n side electrode 207 werewider than the central stripe 209, it would overlie part of the diffusedregion 208 and create an undesirable low resistance conductive pathbetween the electrodes consisting of p-type material in the region 208.Consequently, the width of the active region is directly related to thatof the electrode. Because it is preferable to have the width of theactive region smaller than 2 μm for single transverse mode oscillation,this implies that the n side electrode must be narrower than 2 μm.Common photolithographic production methods do not, however, readilyallow the fabrication of electrodes of a width under 2 μm. As a result,the oscillation mode of semiconductor lasers of the prior art is notvery stable and the threshold current cannot be reduced. Even if it werepossible to produce, by an advanced technology, an electrode of a widtharound 1 μm, positioning such a narrow electrode would be quitedifficult. Moreover, the electrode resistance would be too large toconduct the current required for continuous laser oscillation.

SUMMARY OF THE INVENTION

In view of the foregoing, it is a general aim of the present inventionto provide a semiconductor laser suitable for integration with otherelectronic circuitry and of improved oscillation stability and lowerthreshold current.

Accordingly, it is an object of the present invention to provide asemiconductor laser in which the electrodes are located on the same sideof the substrate and where the minimum width of the active layer is notlimited by the fabrication constraints of the central stripe electrode.

An additional object of the present invention is to provide a method ofproducing a semiconductor laser having such properties.

Other objects and advantages of the present invention will becomeapparent from the detailed description given hereinafter; it should beunderstood, however, that the detailed description and specificembodiments are given by way of illustration only, since, from thisdetailed description, various changes and modifications within thespirit and scope of the invention will become apparent to those skilledin the art.

It is a feature of the present invention that in a buried heterojunctionlaser utilizing a disordered multiquantum-well, a portion of the surfaceof the upper cladding layer located above disordered active layerregions is electrically connected with the upper cladding layer locatedabove the nondisordered active layer area. As a result, the electrodelocated above the nondisordered active layer is no longer confined tothe nondisordered active layer area but can now extend over a portion ofthe surface of the upper cladding layer located above disordered activelayer areas.

According to a method of producing a semiconductor laser of the presentinvention, a lower cladding layer, quantum well active layer, and uppercladding layer of a given conductivity type are successively grown on asemiconductor substrate to form a heterostructure. A portion of theactive layer is then disordered by conducting, from above the uppercladding layer, a solid phase diffusion of impurities of a conductivitytype opposite the conductivity type of the upper cladding layer. Thisdiffusion process also inverts the conductivity type of diffused regionsof the upper cladding layer. Next, the conductivity of a portion only ofthe diffused regions of the surface of the upper cladding layer isre-inverted to the original conductivity type of the upper claddinglayer. Finally, p and n side electrodes are produced on thecorresponding conductivity type regions of the upper cladding layer,thereby completing the fabrication process of a buried heterojunctionsemiconductor laser of the present invention.

According to a preferred method, the region of re-inverted conductivitytype is formed self-aligningly with the nondisordered active region.Following solid phase diffusion, part of the diffusion source isremoved, and a part which had defined one of the diffused regions isleft in place. The part which is left in place is used as a mask for anion implantation operation which re-inverts the conductivity of theunmasked part of the diffused regions.

It is a feature of the invention that the upper cladding layerre-inverted region becomes electrically connected with the uppercladding layer located above the nondisordered active layer area, andthe width of the active layer can be easily reduced without beinglimited by minimum size considerations of the central stripe electrode.

A semiconductor laser having such a narrow active region can, however,be easily produced because of the self-aligning fabrication feature ofthe central stripe electrode. In addition, a semiconductor laser inwhich the active region is small will oscillate in a more stable mannerand require fewer carriers to be injected in the active region area inorder to maintain oscillation. As a result, the threshold current willbe reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a)-1(e) are schematic diagrams showing a method of producing asemiconductor laser as an embodiment of the present invention; and

FIG. 2 is a cross-section exemplifying a prior art buried heterojunctionsemiconductor laser using a disordered superlattice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning now to the drawings, FIGS. 1(a)-1(e) illustrate the major stepsin producing a semiconductor laser according to the present invention.In the illustrated embodiment, a p-type AlGaAs lower cladding layer 202,a multiquantum-well active layer 203, an n-type AlGaAs upper claddinglayer 204, and an n-type GaAs contact layer 205 are successivelydeposited on a semi-insulative GaAs substrate 201 by chemical vapordeposition epitaxy or by molecular beam epitaxy. The term upper claddinglayer is used herein in a broad sense and should be understood toinclude both the AlGaAs layer 204 and, when present, the associated GaAslayer 205.

To disorder the multiquantum-well, a ZnO doped film 101 is used as adiffusion source. The diffusion film 101 and a SiO₂ film 102, the latterbeing used to cap ZnO film 101, are grown on the main surface of n-typeGaAs contact layer 205 by chemical vapor deposition epitaxy or by asputtering method. The preferred thickness of each film 101 and 102 isabout 500 Å. Although the SiO₂ film 102 is not absolutely required, bycapping the diffusion source 101 with SiO₂ film 102 diffusion of Zn intothe ambient is prevented. As a result, diffused regions of higher zincconcentration are obtained.

In the next step, a window 103 in a shape of a stripe approximately 1082 m wide is removed from the SiO₂ and ZnO films, 102 and 101,respectively, using a process such as photolithography and chemicaletching. This window 103, which is bordered on its right side by regions111, 112 of the ZnO diffusion film and SiO₂ capping film, respectively,may be coated over by a dielectric film such as Si₃ N₄ (not shown) inorder to protect the surface of n-type GaAs contact layer 205 asrequired.

When this wafer, having a window is annealed at 700° C. for severalhours in a nitrogen ambient or in an ambient consisting of a mixture ofnitrogen and hydrogen, a so-called solid phase diffusion of Zn from ZnOfilm 101, 111 occurs thereby producing p diffused regions 208. Theportions of the upper cladding layer 204 including contact layer 205within the diffused regions 208 are inverted in conductivity, in theillustrated embodiment from n to p type. In the active layer 203, whichconsists of a multiquantum-well, a smearing of the boundaries of thethin well and barrier GaAs and AlGaAs layers occurs during the solidphase diffusion process, changing the active layer into an AlGaAs layerof averaged composition. This process is called the disordering of themultiquantum-well.

In the region beneath window 103 which is not diffused with pimpurities, the active layer remains a nondisordered multiquantum-wellcreating a central stripe 209 (FIG. 1(c)) which functions as an activeregion. It is seen that portion 114 of the upper cladding layer and acorresponding portion 115 of the contact layer, which portions overliethe central stripe 209, retain their original conductivity type. Evenwhen window 103 is 10 μm wide, and because diffusion occurs in alldirections including into the stripe, diffused regions 208 are expandedin a direction perpendicular to the direction of growth of the layers onsubstrate 201. As a result, by controlling the characteristics of thediffusion process such as diffusion source impurity concentration andannealing conditions, it becomes possible to easily reduce the width ofactive layer 209 to a dimension under 2 μm which is recommended forfundamental mode oscillation.

After this solid phase diffusion, portion 111 of ZnO film 101 which wasused as a diffusion source and portion 112 of SiO₂ film 102 are removedso as to widen window 103. In the illustrated embodiment, film layers111 and 112 are removed, while the portions of film layers 101, 102 onthe left-hand side of window 103 are left in place. It is important tonote that the edge 113 of left in place portions of films 101, 102, byvirtue of the fact that it had defined (in the diffusion process) theextent of the diffused region 208, has a known positional relationshipto the central stripe 209.

In accordance with the invention, means are provided for re-invertingthe conductivity type of a portion only of the surface of the invertedupper cladding layer to its original conductivity type. The re-invertedregion is positioned such that it is in electrical contact with theportion 114, 115 of the upper cladding layer located above the centralstripe 209. Advantageously, a mask 116 comprising left in place portionsof films 101, 102 formed as described above, is used in re-inverting aportion of the surface of the upper cladding layer in a fashion which isself-aligning with the central stripe 209.

More particularly, in the illustrated embodiment, after removal ofdiffusion source portions 111, 112 as described above, Si ions areimplanted into the upper cladding layer 204 and contact layer 205 usingleft in place portions of films 101, 102 as a mask 116. As a result,this ion implantation re-inverts the conductivity of a portion of thesurface of diffused regions 208 to n-type (in the illustratedembodiment, region 105 extending rightward from edge 113), while leavingthe upper cladding layer region on the left-hand side of edge 113 ofp-type conductivity. It produces, self-aligningly with active region209, an n-type region 105, in electrical contact with the upper claddinglayer portion 114, 115 located above nondisordered active region 209 asshown in FIG. 1(d).

Finally, after removing mask 116 in the area where the diffused contactlayer 205 meets the re-inverted region 105, the contact layer is etchedto form a groove in order to separate p- and n-type contact layerregions. To complete the fabrication process, p and n side electrodes206 and 207 are then produced on the corresponding p-type and n-typeregions of the GaAs contact layer.

Because n-type region 105 overlies not only the central stripe 209 butalso at least a portion of one of the diffused regions 208, andrecalling that it is in electrical contact with the upper cladding layerportion 114, 115 which overlies the central stripe, it is seen that then side electrode is no longer confined to the width of the centralstripe. As can be appreciated upon reference to FIG. 1(e), the n-sideelectrode 207 (like the p-side electrode 206) can be of a size largerthan the central stripe 209, and can thus be made of a size whichfacilitates production, without running the risk of shorting the activeregion through the p-type disordered region as encountered in the FIG. 2prior art semiconductor laser.

Turning now to the operation of the semiconductor laser of the presentinvention, when a voltage is applied between the p side and n sideelectrodes with the p side electrode positive with respect to the nside, carriers are injected into the active region to cause laseroscillation under the same principle as that of prior art semiconductorlasers. The conductive path includes the p-side electrode 206, thep-type diffused regions of the contact layer 205, upper cladding layer204 and p-type lower cladding layer 202 to the periphery of the activeregion 209. The p-n junction is formed between those p-type layers andn-type region 114 of the upper cladding layer. The conductive path iscompleted through the enlarged re-inverted region 105 of the uppercladding and contact layers 204, 205 to the enlarged n-side electrode207. As in the FIG. 2 embodiment, the diffusion voltage of the p-njunction at the periphery of the active region 209 is lower than that ofthe p-n junction between the opposite conductivity type regions in theupper cladding layer. As a result, current will flow through the formerand carriers will be injected into the active region. In the describedembodiment of the invention, because the width of the active region canbe made very narrow without being restricted by minimum widthrequirements for the n side electrode, fundamental transverse modeoscillation easily occurs and the threshold current is reduced to about1 mA. In addition, the fact that the p side and n side electrodes arelocated on the same main surface and at about the same level allowsintegration of this semiconductor laser with other electronic circuitry.

In the above-illustrated embodiment, the active layer is amultiquantum-well although it may instead consist of a single quantumwell or a superlattice. Furthermore, the proportions in Al compositionof the p-type and n-type AlGaAs cladding layers, between which theactive layer is sandwiched, do not necessarily have to be constantwithin each cladding layer but may vary gradually, along the directionof growth of the layers, as in a graded type laser.

In the illustrated embodiment, a GaAs series laser is described, but thepresent invention is applicable as well to an InP series laser or to aseries laser of a different material.

In the illustrated embodiment, Zn diffusion from a zinc oxide doped filmis used as an example of solid phase diffusion. However, solid phasediffusion from a silicon film of n-type impurities can be used in asemiconductor laser in which the conductivity type of each layer is thereverse of that shown in the illustrated embodiment.

The AlGaAs lower cladding layer may be of n-type conductivity instead ofp-type as in the illustrated embodiment. This lower cladding layer mayalso be of a semi-insulative type. In this case, the injection ofcarriers occurs only from the upper cladding layer as carriers cannot beinjected from the semi-insulative layer. However, effects almost similarto those obtained when the lower cladding layer is not of thesemi-insulative type should result.

In the above-illustrated embodiment, a GaAs contact layer 205 withseparation groove 106 is provided on the upper cladding layer. However,this GaAs contact layer 205, although preferred, is not mandatory. Whenno contact layer is provided, the value of the contact resistancebetween each electrode and its respective conductivity region is higherbut no separation groove is necessary.

The foregoing description of the operation of a semiconductor laseraccording to the present invention shows that, in a buriedheterojunction laser utilizing the disordering of a multiquantum-well, aportion of the surface of the upper cladding layer located above thedisordered active layer regions is electrically connected with the uppercladding layer located above the nondisordered central stripe. As aresult, the central stripe width can be reduced independently of that ofthe corresponding electrode, which can, therefore, be of a large size.Additionally, the oscillation of the semiconductor laser is renderedmore stable and the required threshold current is reduced. Moreover,because both p and n side electrodes are located on the upper surface ofthe chip, this configuration is appropriate for integration with otherelectronic circuitry.

According to a method of producing a semiconductor laser of the presentinvention, a portion of an active layer is disordered by conducting,from above an upper cladding layer, solid phase diffusion of first typeconductivity impurities. A portion of the surface of the upper claddinglayer, located above the disordered active layer regions and which wasinverted to first type conductivity during the above-described solidphase diffusion, is re-inverted to second type conductivity and iselectrically connected with the upper cladding layer located above thenondisordered central stripe. This re-inversion process is restricted tocertain areas of the upper cladding layer by using as a mask the onepart of the diffusion source which had defined one of the diffusedregions and which was left in place after the diffusion step so as toform a region of inverted conductivity type self-aligningly with thenondisordered active region.

Accordingly, this production method provides a semiconductor laser inwhich both p and n side electrodes are located on the upper surface ofthe chip and where the width of the active region is reduced withoutbeing any longer limited by minimum size considerations of theconnecting electrode. The semiconductor laser obtained through thisprocess can further be produced easily self-aligningly and oscillates ina stable manner with low threshold current. It is also in aconfiguration appropriate for integration with other electroniccircuitry such as FET devices which could be used to modulate lasercurrent and, therefore, the light being emitted by the laser. Moreover,the electrode which is connected to the conductivity area located abovethe nondisordered active layer, can be of a significant size as it nolonger has to match the width of the semiconductor laser active region.

What is claimed is:
 1. A buried heterojunction semiconductor lasercomprising:(a) a quantum well active layer; (b) upper and lower claddinglayers confining said active layer; (c) diffused regions of a firstconductivity type in the upper cladding layer, said diffused regionsextending into the active layer to define in the active layer disorderedregions and a nondisordered central stripe; (d) second conductivity typesurface regions of said upper cladding layer partially extending oversaid diffused regions and in electrical contact with the upper claddinglayer located above said nondisordered central stripe; and (e) a pair ofelectrodes on the upper cladding layer, one electrode being inelectrical contact with the first and the other in contact with thesecond conductivity type regions of the upper cladding layer.
 2. Theburied heterojunction laser as defined in claim 1, wherein theelectrodes are wider than the nondisordered central stripe.
 3. Theburied heterojunction laser as defined in claim 1, wherein saiddisordered regions contain diffused impurities of a conductivity typeopposite from the original conductivity type of the upper claddinglayer.
 4. The buried heterojunction laser as defined in claim 1, whereinthe upper cladding layer includes a contact layer at the upper surfacethereof, said contact layer being divided into two regions correspondingto and having the same conductivity type as the regions of the uppercladding layer.
 5. The buried heterojunction laser as defined in claim4, wherein the contact layer includes a groove separating the first andsecond type conductivity regions of said contact layer.
 6. A buriedheterojunction semiconductor laser comprising:(a) a quantum well activelayer; (b) upper and lower cladding layers confining said active layer;(c) the active layer having a central stripe bounded by a pair ofdisordered regions; (d) the upper cladding layer being divided into tworegions of opposite conductivity type, one region overlying andextending beyond said central stripe and the other region being adjacentto the stripe; (e) electrodes of a size larger than the central stripeon said two upper cladding layer regions and in electrical contacttherewith.